DC power supply and battery charge system

ABSTRACT

A power supply for providing full time electrical power to a customer premises telecommunications hub. The supply includes an AC to DC power converter for converting power from the AC power grid to a DC voltage selected to maintain a backup battery at float voltage. The converter includes a first rectifier section for generating an unregulated DC voltage. This voltage is switched through the primary of an isolation transformer by a pulse width modulated voltage controller. The output of the transformer is connected to a second rectifier circuit and filter to produce a regulated DC output voltage. The regulated voltage is connected to a voltage correction circuit through a divider including a temperature compensator so that the feedback to the voltage controller causes the regulated output voltage to follow the battery float voltage at all temperatures. The unregulated DC voltage is coupled to the voltage controller current limiting input to protect the power supply at high input voltages.

This is a divisional application of U.S. patent application Ser. No.09/675,585, filed Sep. 29, 2000, now U.S. Pat. No. 6,297,620 herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to combination power supplies and batterychargers, and more particularly to power supplies for telecommunicationshubs located on business or customer premises which provide power tooperate the hub while maintaining backup batteries at float charge levelcompensated for ambient temperature changes.

BACKGROUND OF THE INVENTION

Traditionally, telephony communications within the United States werehandled by the public switched telecommunications network (PSTN). ThePSTN can be characterized as a network designed for voicecommunications, primarily on a circuit-switched basis, with fullinterconnection among individual networks. The PSTN network is largelyanalog at the local loop level, digital at the backbone level, andgenerally provisioned on a wireline, rather than a wireless, basis. ThePSTN includes switches that route communications between end users.Circuit switches are the devices that establish connectivity betweencircuits through an internal switching matrix. Circuit switches setconnections between circuits through the establishment of a talk path ortransmission path. The connection and the associated bandwidth areprovided temporarily, continuously, and exclusively for the duration ofthe session, or call. While developed to support voice communications,circuit switches can support any form of information transfer (e.g.,data and video communications).

In a traditional PSTN environment, circuit switches include centraloffice (CO) exchanges, tandem exchanges, access tandem exchanges, andinternational gateway facilities. Central offices, also known asexchanges, provide local access services to end users via local loopconnections within a relatively small area of geography known as anexchange area. In other words, the CO provides the ability for asubscriber within that neighborhood to connect to another subscriberwithin that neighborhood. Central offices, also known as end offices,reside at the terminal ends of the network. In other words, COs are thefirst point of entry into the PSTN and the last point of exit. They arealso known as class 5 offices, the lowest class in the switchinghierarchy. A class 5 telephone switch communicates with an analogtelephone using the analog telephony signals in the well-known analogformat. The class 5 telephone switch provides power to the telephone;detects off-hook status of the telephone and provides a dial tone inresponse; detects dual-tone multi-frequency signals from the caller andinitiates a call in the network; plays a ringback tone to the callerwhen the far-end telephone is ringing; plays a busy tone to the callerwhen the far-end telephone is busy; provides ring current to thetelephone on incoming calls; and provides traditional telephone servicessuch as call waiting, call forwarding, caller ID, etc.

In an effort to increase the amount and speed of information transmittedacross networks, the telecommunications industry is shifting towardbroadband packet networks which are designed to carry a variety ofservices such as voice, data, and video. For example, asynchronoustransfer mode (ATM) networks have been developed to provide broadbandtransport and switching capability between local area networks (LANs)and wide area networks (WANs). The Sprint ION network is a broadbandnetwork that is capable of delivering a variety of services such asvoice, data, and video to an end user at a residential or businesslocation. The Sprint ION network has a wide area IP/ATM or ATM backbonethat is connected to a plurality of local loops via multiplexors. Eachlocal loop carriers ATM over ADSL (asymmetric digital subscriber line)traffic to a plurality of integrated service hubs (ISHs), which may beat either residential or business locations.

An ISH is a hardware component that links business or residential userdevices such as telephones and computers to the broadband, wide areanetwork through a plurality of user interfaces and at least one networkinterface. A suitable ISH is described in co-pending U.S. patentapplication Ser. No. 09/226,575 entitled “Multi-Services CommunicationsDevice,” filed on Jan. 7, 1999 (Sprint docket number 1246), which isincorporated by reference herein in its entirety. The network interfacetypically is a broadband network interface such as ADSL, T1, or HDSL-2.Examples of user interfaces include telephone interfaces such as plainold telephone system (POTS) ports for connecting telephones, faxmachines, modems, and the like to the ISH; computer interfaces such asethernet ports for connecting computers and local area networks to theISH; and video ports such as RCA jacks for connecting video players,recorders, monitors, and the like to the ISH.

In providing telephony services over a broadband network, the ISHconnects a telephone in the customer's premises to a network elementsuch as a service manager. This connection between the telephone and thenetwork element is typically an ATM connection, which is much differentthan the traditional analog line to the local switch. ATM connectionsusually do not support analog telephony signals, such as off-hook, dialtone, and busy signals. Therefore, the ISH must provide many of thetelephony functions traditionally provided by the telephone providercentral office such as detect off-hook conditions, on-hook connections,and digits as well as provide the telephones with dial tone, ringcurrent, ringback, and busy signals. The terms off-hook and off-hookcondition as used herein are generic terms meaning that a user device(whether telephone, facsimile machine, modem, etc.) connected to atelephone line is attempting to access and use the line.

Another example of such a central office function being provided by theISH is backup power. Traditionally in cases of power grid failure, thecentral office provides backup power to customers' telephones throughuse of an industrial-strength, petroleum-fueled backup generator. Sinceit is not economical to equip each customer with a backup generator, anISH must be equipped with a back-up power supply, which is typically abattery pack, to maintain power to the system in cases of power gridfailure.

The ISH must include a power supply to support the telephony functions(off hook, dial tone, etc.) and to keep the battery pack in chargedcondition so that it can provide backup power for as long as possible inthe event of power grid failure. The power supply of the ISH should beas simple as possible to be cost effective; and yet it is desirable thatthe power supply be able to operate continuously, use as little power aspossible when the power grid fails, and provide high voltage isolationof the user equipment from the power grid.

SUMMARY OF THE INVENTION

A power supply according to the present invention includes an AC to DCpower converter for converting AC power from the power grid to DC powerfor use in the ISH and a sealed lead acid battery connected directly tothe DC output of the power converter. The power converter includes afirst rectifier section for converting AC power from the power grid intoan essentially unregulated first DC voltage. An isolation transformerhas a primary coil connected to the first DC voltage and to a pulsewidth modulated voltage controller. The secondary of the isolationtransformer is connected to a second rectifier section for producing aregulated second DC voltage at an output which is connected to thebattery. A voltage comparator has an input connected to the second DCvoltage by a divider circuit which includes a temperature sensitivedevice which adjusts the feedback voltage in proportion to batterytemperature. The output of the voltage comparator drives the voltagecontrol input of the voltage controller to maintain the regulated DCoutput voltage at the level needed to maintain the battery at floatvoltage over the operating temperature range. A current limit input tothe voltage controller is provided with an input which is a combinationof the primary winding current level and the first DC voltage level, tomaintain maximum power availability level without overheating the powersupply.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a power supply according to the presentinvention; and

FIGS. 2A and 2B together provide an electrical schematic diagram of thecircuitry of an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, there is provided a block diagram illustratingthe primary elements of a power supply according to the presentinvention. Block 10 represents a source of AC power, which for many userapplications will be 117 volt 60 Hertz AC power from a residential powergrid which is standard in the United States. The preferred embodiment,however, is adapted for receiving input voltages from 90 to 275 voltsand at frequencies from 40 to 440 Hertz, to accommodate power grids inessentially all countries. The input power passes through a primaryprotection and EMI control section 12 to reduce power spikes and noise.The power then passes through block 14 where additional protection andrectification is provided. A first DC voltage is provided between outputlines 16 and 18. This first DC voltage is not closely regulated and willvary depending on input voltage. An high frequency isolation transformer20 is connected to the outputs 16 and 18 and to a pulse width modulatedvoltage controller 22. The secondary of transformer 20 is connected to arectification and filtering section 24 which provides a closelyregulated DC voltage across its outputs 26. The regulated DC voltage isfed back to a voltage comparator 28. Comparator 28 provides a voltagecontrol signal through optical feedback 30 to voltage controller 22.

Voltage controller 28 also receives a temperature compensation input online 32. This signal is generated by a thermistor 34 located nearbattery 36. Battery 36 is connected through backplane interface 38 tothe regulated voltage on lines 26. Thermistor 34 is likewise connectedthrough the backplane 38 to the control circuitry. Backplane 38 is aprinted circuit board with a number of sockets for receiving andinterconnecting the various printed circuit boards which comprise atelecommunications hub. Every board connected to the backplane requireselectrical power. The power supply of the present invention providesthis power from the input 10 so long as the power grid is working. Ifthe power grid fails, the battery 36 is hard wired to the backplane andcontinues to provide the needed power to allow continued operation ofthe telecommunications hub.

Prior art systems such as that shown in U.S. Pat. No. 4,663,580typically provide switches and necessary control circuitry to isolate aback up battery from a system requiring power until the power gridactually fails. Other systems such as that shown in U.S. Pat. No.5,623,195 provide additional circuitry to avoid thermal runaway orovercharging of batteries which have been discharged. The presentinvention provides very simple cost effective circuitry in place of themore complicated prior art systems.

Under normal conditions, that is, when the power grid is working, it isimportant to keep the battery 36 fully charged, but not overcharged.This requires a closely regulated voltage which is compensated forbattery temperature. The float, or fully charged voltage of lead acidbatteries is known to vary with temperature. As temperature increases,the battery voltage decreases. If a constant supply voltage is appliedto a battery, the charging current will increase as temperature rises.In a sealed lead acid battery, it is important to prevent overchargingbecause of its limited capacity to recombine oxygen and hydrogen whichare produced by excess current. In the present invention, the thermistor34 is placed near the battery 36. Its resistance changes withtemperature and provides a feedback signal to adjust the regulatedoutput voltage so that it matches the float voltage of the battery 36.As explained below, by proper choice of components the voltage controlcan be accurate within one percent.

If the “unusual” condition of power grid failure occurs, then power issupplied by the battery 36. The battery will of course discharge duringthe power outage. Upon restoration of power from the power grid, thebattery 36 will begin recharging. Depending on the state of discharge,the battery could draw significant currents, especially if the inputsource 10 is a high voltage source. To prevent overstressing componentsin this situation, the present invention provides a simple currentlimiting protection arrangement. In FIG. 1, resistor R28 is an in-linecurrent sensing resistor used to provide a current limiting signal tovoltage controller 22. An additional resistor R2 is connected from thepositive unregulated DC voltage line 16 to the current sense input. Asthe voltage on line 16 increases, the signal from resistor R2 limitscurrent through the primary of transformer 20 to protect components fromoverheating and possible failure.

A detailed electrical schematic diagram of an embodiment of the presentinvention is provided in FIGS. 2A and 2B. Components which are alsoindividually shown in FIG. 1 are identified with the same numbers inFIGS. 2A and 2B.

In FIG. 2A, the AC power grid input 10 is illustrated with the standardUS power plug configuration. The primary protection (box 12 of FIG. 1)is provided by capacitors C1, C2 and C3. Secondary protection andrectification (box 14, FIG. 1) is provided by transformer T1, fuse F1,thermistor R1 and full wave diode bridge D1. The first DC voltageappears across lines 16 and 18 and is smoothed by capacitor C4 and peaklimited by zener diodes D3 and D7.

Primary winding 40 of isolation transformer 20 is connected betweenpower lines 16 and 18 in series with power transistor Q3 and currentsensing resistor R28. A secondary winding 42 together with diodes D6 andcapacitors C10 and C16 provide operating voltage to pulse widthmodulated voltage controller 22. The driver output of controller 22 isconnected through resistor R12 to transistor Q3. Secondary windings 44of transformer 20 are connected to diode D8 and capacitors C6 and C5 toprovide rectification and filtering (block 24, FIG. 1) for the regulatedoutput DC power on line 26.

FIG. 2B illustrates the voltage control section (block 28, FIG. 1) andother parts of the power supply. Line 26 connects the regulated DC powerto the backplane interface 38, and through it to battery 36 and allother systems which are operated by this power. The thermistor 34 isconnected through the backplane interface to a resistor divider stringcomprising resistors R7, R5, R4, and R15 connected in series betweenline 26 and ground. Thermistor 34 is connected in parallel with resistorR5. The voltage at the junction of resistors R4 and R15 is applied tothe positive input of op-amp, operational amplifier, 46. This amplifiercompares the input voltage to an internal reference and provides acontrol signal through transistor Q6 and optical isolator ISO1 to thecontrol input of voltage controller 22. The resistance of thermistor 34changes with the temperature of battery 36, which in turn changes thefeedback signal to op-amp 46, which causes the controller 22 to adjustthe output voltage on line 26 to match the float voltage of battery 36.With the components specified in FIG. 2B, the output voltage iscompensated at the rate of minus 3 millivolts per degree centigrade percell. For the 12 volt battery of this embodiment, the compensation istherefore minus 18 millivolts per degree centigrade. The parallelcombination of the fixed resistor R5 causes some desirable deviationfrom this compensation rate at the high end.

In this embodiment, op-amp 46 is a part number LTC1541 manufactured byLinear Technology Corporation. Use of this part, or an equivalent part,is important for two reasons. It contains a voltage reference with a0.4% accuracy. When this is combined with the resistor string R7, R5, R4and R15 having a total accuracy of 0.5%, the voltage control is accuratewithin 1%. With this level of accuracy, the battery 36 can bepermanently connected to the backplane interface and kept at full chargewithout overcharging which would shorten its lifetime and reduce itscapacity. The LTC 1541 device uses very little power, requiring onlyabout 5 microamps of current. The resistor string specified in FIG. 2Buses about 15 microamps. At these low power levels, there is no need todisconnect the voltage control circuitry when the power grid fails, evenif the following system circuits are switched off, such as for storage.

As discussed above, the present invention also includes a currentlimiting circuit to protect the power supply as shown in FIG. 2A.Resistor R28 is connected in series with power transistor Q3 to sensethe current levels in primary winding 40 of transformer 20. The voltageacross resistor R28 is coupled to the current sensing input of voltagecontroller 22 through resistor R30. In the present invention a secondinput is provided to the current sense input. Resistor R2 is connectedfrom the current sense input to line 16. As the voltage on line 16increases, the peak current levels in primary 40 of transformer 20 aredecreased. This prevents damage which might otherwise occur at highinput voltages, while allowing use of full supply power capacity overthe full operational range.

In FIG. 2B there is also illustrated a low voltage shutdown circuit. Itincludes a voltage comparator 48 which is physically part of the sameLTC1541 device which contains op-amp 46. Comparator 48 has a positiveinput connected to the junction of resistors R4 and R5 and a negativeinput connected to a reference voltage. With the values shown,comparator 48 will generate a discharge shutdown signal when the batteryvoltage drops to about nine volts. At this level, at least one cell ofbattery 36 is fully discharged. Further discharging would probably causepermanent damage to the battery. The discharge shutdown signal iscoupled by the backplane interface to the other devices plugged into thebackplane. In response to the shutdown signal the other devices shouldgo into an inactive state and essentially stop drawing power from thebattery.

While the present invention has been illustrated and described withreference to specific circuits and methods of operation, it is clearthat various modifications thereof and substitution of parts may be madewithin the scope of the invention as defined by the appended claims.

What is claimed is:
 1. A power supply comprising: a first AC powerconverter having an input for connection to an AC power grid and atleast one rectifier for producing a first DC voltage at its output; anisolation transformer having a primary winding connected to said firstDC voltage and having a secondary winding; a second AC power converterhaving an input connected to the secondary winding and at least onerectifier for providing a second DC voltage at an output; a voltagecontroller having a voltage control input, a battery connected to theoutput of said second AC power converter, a temperature sensing devicelocated near the battery, and a low power voltage comparator having aninput connected to the output of said second AC power converter througha voltage divider including the temperature sensing device and having anoutput coupled to the voltage controller voltage control input, saidbattery and the output of said second AC power converter connected to aDC power bus in a backplane in a customer premises telecommunicationshub.
 2. A power supply according to claim 1 wherein: said voltagecontroller further comprises a current switching output connected to theisolation transformer primary winding, and a current limiting input forreceiving a signal to limit current through said primary winding to safelevels.
 3. A power supply according to claim 2 further comprising: acurrent sensing circuit connected to said voltage controller currentlimiting input, including a resistor connected to said first DC voltage.4. A power supply according to claim 3 wherein: said current sensingcircuit further comprises means for sensing current through saidisolation transformer primary winding.
 5. A power supply for a customerpremises telecommunications hub comprising: an AC to DC power converterhaving an input for receiving AC power from a commercial power grid andan output connected to a DC power bus of the telecommunications hub andincluding a voltage controller having a voltage control input; a sealedlead acid battery connected to the DC power bus; a temperature sensingdevice located near the battery; and a low power voltage comparatorhaving an input connected to the DC power bus through a voltage dividerincluding the temperature sensing device and having an output coupled tothe voltage controller voltage control input.
 6. A power supplyaccording to claim 5 wherein: the voltage on said DC power bus iscontrolled at the float voltage of said battery.
 7. A power supplyaccording to claim 5 wherein: the voltage on said DC power bus changeswith temperature at the rate of about minus three millivolts per cellper degree centigrade.
 8. A power supply according to claim 5 wherein:said voltage comparator and said voltage divider have an overallaccuracy of less than about one percent.
 9. A power supply for acustomer premises telecommunications hub comprising: an AC to DC powerconverter having an input for receiving AC power from a commercial powergrid and an output connected to a DC power bus of the telecommunicationshub and including a voltage controller having a voltage control input; abattery connected to the DC power bus; a temperature sensing devicelocated near the battery; a low power voltage comparator having an inputconnected to the DC power bus through a voltage divider including thetemperature sensing device and having an output coupled to the voltagecontroller voltage control input; and a low voltage shutdown circuithaving an input connected to said DC power bus and an output forproviding a signal indicating that the battery voltage has dropped belowa preselected level.
 10. A power supply according to claim 9 wherein:said preselected level is a level below which permanent damage to saidbattery is likely to occur.
 11. A power supply comprising: a first ACpower converter having an input for connection to an AC power grid andat least one rectifier for producing a first DC voltage at its output;an isolation transformer having a primary winding connected to saidfirst DC voltage and having a secondary winding; a pulse width modulatedvoltage controller connected to the isolation transformer primarywinding having a current limiting input to limit current through saidprimary winding to safe levels; a current sensing circuit connected tosaid voltage controller current limiting input, including a resistorconnected to said first DC voltage; a second AC power converter havingan input connected to the secondary winding and at least one rectifierfor providing a second DC voltage at an output; a battery connected tothe output of said second AC power converter; a temperature sensingdevice located near the battery, and a low power voltage comparatorhaving an input connected to the output of said second AC powerconverter through a voltage divider including the temperature sensingdevice and having an output coupled to a voltage control input of saidvoltage controller, said battery and the output of said second AC powerconverter connected to a DC power bus in a backplane in a customerpremises telecommunications hub.
 12. A power supply according to claim11 wherein; said current sensing circuit further comprises means forsensing current through said isolation transformer primary winding. 13.A power supply according to claim 11 wherein; said battery is a sealedlead acid battery.
 14. A power supply according to claim 13 wherein:said second DC voltage is controlled at the float voltage of saidbattery.
 15. A power supply according to claim 13 wherein: said secondDC voltage changes with temperature at the rate of about minus threemillivolts per cell per degree centigrade.
 16. A power supply accordingto claim 11 wherein; said voltage comparator and said voltage dividerhave an overall accuracy of less than about one percent.
 17. A powersupply according to claim 11 further comprising; a low voltage shutdowncircuit having an input connected to said battery and an output forproviding a signal indicating that the battery voltage has dropped belowa preselected level.
 18. A power supply according to claim 17 wherein:the output of said low voltage shutdown circuit is connected to saidbackplane, whereby circuitry comprising said customer premisestelecommunications hub connected to said backplane may be shut down toprevent over discharge of said battery.
 19. A power supply according toclaim 17 wherein: said preselected level is a level below whichpermanent damage to said battery is likely to occur.
 20. A power supplyaccording to claim 11 wherein; said first AC power converter input isadapted for connection to an AC power grid having a voltage of fromabout 90 to about 275 volts and a frequency of from about 40 to about440 Hertz.